Acyl coenzyme A (acyl-CoA) thioesterases (ACHs) are enzymes that cleave thioester bonds between CoA and a fatty acyl group. These enzymes, also referred to as acyl-CoA hydrolases, have been identified in all animal and bacterial organisms studied to date, including E. coli and humans. Eukaryotic cells contain various isoforms of these enzymes, which are located in various organelles, most notably mitochondria, peroxisomes, and the endoplasmic reticulum, as well as the cytosol. Although their true physiological role is not currently understood, studies conducted in yeast have indicated that deletion of the peroxisomal form leads to decreased growth when oleic acid is provided as the carbon source. Other studies in rats indicate that other isoforms exhibit increased expression under conditions where fatty acids are being broken down (e.g., during fasting periods). Overall, the experimental evidence gathered to date indicates that these enzymes have a role in lipid oxidation and may be involved in regulation of the CoA pool within cells. Thus, there remains a need for purified forms of these enzymes, as well as nucleic and polypeptide sequences, such that the function of these enzymes can be further elucidated and methods developed for regulation of fatty acid metabolism.